The invention described herein was made in the performance of official duties by an employee of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon.
1. Field of the Invention
The invention relates generally to GPS positioning systems, and more particularly to a system that minimizes the error associated with generating a GPS position of vehicle traveling under the surface of the water.
2. Background of the Invention
Few systems exist for tracking or obtaining position information for vehicles that operate under water. Furthermore, existing technologies either cannot operate automatically or have large errors imbedded in their position fixes. For example, laser-based optical trackers are complex systems that require substantial amounts of set-up time and maintenance thereby making them impractical for use with autonomous underwater vehicles. An approach for automatically obtaining position information of an underwater vehicles uses a Global Positioning System (GPS) receiver mounted on a float which is towed behind the vehicle with a fixed-length tether. However, errors associated with this method are substantial as is explained below.
Since the deactivation of Selective Availability (SA) on May 1, 2000, GPS receivers are capable of obtaining position fixes with a circular error of approximately 10 meters. This error can be further reduced through the use of differentially corrected GPS (DGPS). Differential corrections are generated at a fixed site and transmitted, typically using the Radio Technical Commission for Maritime Services (RTCM) SC 104 standard, to rover DGPS receivers. The rover DGPS receivers then combine the data received from the GPS satellites and the RTCM data to generate the DGPS position fix, which can have less than 1-foot of error when the corrections are generated at a nearby site.
The current approach utilizing DGPS to obtain a position fix on a vehicle operating underwater will now be explained with the aid of FIG. 1 where a vehicle 10 has a fixed-length tether 12 coupled thereto. Tether 12 has a float 14 coupled to its outboard end. Mounted onboard float 14 is a DGPS receiver 16 and a GPS antenna 18 for receiving GPS signals. Vehicle 10 is operating at a depth D and the length of tether 12 being towed is L. Placing GPS antenna 18 and receiver 16 on float 14 can provide fixes that contain less than a foot of circular error relative to the position of float 14 when locally generated differential corrections are used. However, with respect to the position of vehicle 10, there is significant error resulting from the xe2x80x9cwatch circlexe2x80x9d, the radius of which is defined as the horizontal distance W between vehicle 10 and float 14. Watch circle error is the result of a fixed length tether being used to tow a float in water depths less than the length of the tether. The error caused by this effect can be very significant. For example, if tether length L is 15-feet and vehicle 10 is operating at a depth of 10 feet, the resulting watch circle error W can be as much as 11.18 feet.
Accordingly, it is an object of the present invention to provide a system for generating a position fix for a vehicle operating under the surface of the water.
Another object of the present invention is to provide a system that reduces watch circle errors when generating a GPS position fix for a vehicle operating under the surface of the water.
Still another object of the present invention is to provide a system for automatically generating and updating a position fix for a vehicle operating under the surface of the water.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a system generates an approximate GPS position of a vehicle traveling under the surface of the water. A tether is coupled on one end thereof to the vehicle and a plurality of floatable structures are coupled to the tether at spaced-apart positions therealong. A position determination system is typically positioned at either the one floatable structure that is furthest away from the vehicle or onboard the vehicle. A plurality of GPS antennas are provided with at least one GPS antenna being coupled to each remaining one of the floatable structures that reside between the vehicle and the one floatable structure that is furthest away from the vehicle. Each GPS antenna is capable of receiving GPS signals when at the surface of the water. Each GPS antenna is also coupled to the position determination system. The position determination system uses the GPS signals received at the GPS antenna that is nearest the vehicle to generate a global position fix that approximates a GPS position of the vehicle.